A campus-wide letter Thursday didn't specifically reference the photos or name the Chi Omega sorority, which has apologized. But Erickson says it became clear recently that some students celebrated Halloween in costumes and "in a manner that offended others."
One sign in the photo of sorority sisters says "will mow lawn for weed + beer." Two women holding signs wore fake mustaches. Others wore sombreros.
Erickson said the "disturbing behaviors" were protected by free speech, but hoped lessons would be learned.
Later Thursday, Chi Omega's national chapter said on its website the Penn State sorority would go on probation starting Monday.